RF communications systems are already known for transmitting data from an endpoint device to a receiver or intermediate transceiver. In a home security application, a tripped sensor at a window relays an RF message to a central gateway, which in turn relays a message via phone lines to a security-monitoring center. In another application, RF data communications are used to upload or download data via telemetry from an implantable medical device. In both of the above-described applications, the RF data stream is not interfered with because of the close proximity of the transmitting and receiving devices or the lacking presence of interfering devices.
RF communications systems are also being implemented to provide wireless connections to a communications network or to access an Internet Service Provider (ISP). In one such application, wireless ISPs (WISPs) provide internet service to those customers that do not have access to cable or to high speed DSL (digital subscriber line) internet service because the major cable service provider in the area has not laid the “last mile” of cable (or the telephone operating company does not provide DSL service) to the customer. WISP systems typically operate with low power R.F. in the 900 MHz range.
Wireless automatic meter reading systems are also well known. Typically, a utility meter is provided with a battery-powered encoder that collects meter readings and periodically transmits those readings over a wireless network to a central station. The power limitations imposed by the need for the encoder to be battery powered and by regulations governing radio transmissions effectively prevent direct radio transmissions to the central station. Instead, wireless meter reading systems typically utilize a layered network of overlapping intermediate receiving stations that receive transmissions from a group of meter encoders and forward those messages on to the next higher layer in the network as described, for example, in U.S. Pat. No. 5,056,107. These types of layered wireless transmission networks allow for the use of lower power, unlicensed wireless transmitters in the thousands of encoder transmitters that must be deployed as part of a utility meter reading system for a large metropolitan area.
In 1985, as an attempt to stimulate the production and use of wireless network products, the FCC modified Part 15 of the radio spectrum regulation, which governs unlicensed devices. The modification authorized wireless network products to operate in the industrial, scientific, and medical (ISM) bands using spread spectrum modulation. The ISM frequencies that may be used include 902 to 928 MHz, 2.4 to 2.4835 GHz, and 5.725 to 5.850 GHz. The FCC allows users to operate wireless products, such as utility metering systems, without obtaining FCC licenses if the products meet certain requirements. This additional flexibility in the use of the frequency spectrum eliminates the need for the user organizations to perform cost and time-consuming frequency planning to coordinate radio installations that will avoid interference with existing radio systems.
Spread spectrum modulators use one of two methods to spread the signal over a wider area. The first method is that of direct sequence spread spectrum, or DSSS, while the second is frequency hopping spread spectrum, or FHSS. DSSS combines a data signal at the sending station with a higher data rate bit sequence, which many refer to as a chipping code (also known as a processing gain). A high processing gain increases the signals resistance to interference. FHSS, on the other hand, relies on the distribution of a data signal randomly hopped across a number of defined frequency channels to avoid interference. While DSSS has potentially higher data transmission rates than FHSS, DSSS has been much more costly than FHSS, has had higher power consumption, and is more susceptible to noise.
FHSS, on the other hand, operates by taking the data signal and modulating it with a carrier signal that hops from frequency to frequency as a function of time over a wide band of frequencies. A hopping code determines the frequencies the radio will transmit and in which order. To properly receive the signal, the receiver must be set to the same hopping code and listen to the incoming signal at the right time and correct frequency. If the radio encounters interference on one frequency, then the radio will retransmit the signal on a subsequent hop on another frequency. Because of the nature of its modulation technique, FHSS can achieve up to 2 Mbps data rates. It is possible to have operating radios use FHSS within the same frequency band and not interfere, assuming they each use a different hopping pattern. The frequency hopping technique reduces interference because an interfering signal from a narrowband system will only affect the spread spectrum signal if both are transmitting at the same frequency and at the same time.
The aggregate interference using FHSS should be very low, resulting in little or no bit errors. However, depending on the frequency that other low power RF communication systems may be operating, some signal collisions may occur and data may be lost.
With respect to meter reading applications in the context of wireless radio networks there is a potential for collisions between transmissions of a large number of units concentrated in a relatively small area. This problem is particularly acute, for example, in the context of sub-metering applications which involve the allocation of utility usage readings over a large number of units in an apartment, high rise, office building or other dwelling where multiple utility accounts may be located in the same building or in the same building complex. Sub-metering applications also tend to present severe challenges in terms of installation and operation due to structures limiting or blocking effective antenna coverage.
In 2002, the FCC further modified Part 15 of the radio spectrum regulation to provide for the introduction of new digital transmission technologies, thereby eliminate any regulatory distinction between direct sequence spread spectrum (DSSS) systems and systems using other forms of digital modulation. Digital modulation systems will be subject to the same power output maximum, 1 Watt, and power spectral density limits, 8 dBm per 3 kHz, as in DSSS systems but will not be subject to the same processing gain constraints as in prior RF communication systems.
In view of the above, there is a need for an RF communications system that wirelessly communicates data without encountering undue interference problems with other devices and that complies with revised Part 15.247 of the FCC rules governing unlicensed, spectrum-sharing devices. There is also a need for digital modulation techniques that preserve the battery-life of end-point transmitters, that can eliminate intermediate repeaters, and that enable improved signal collision avoidance.